With the prior art disc brake device of this type, braking is achieved by hydraulic pressure from a master cylinder in cooperation with a mechanical action from a hand brake lever, etc. To make adjustment in response to the displacement of the piston, an automatic braking-gap adjusting mechanism includes a built-in automatic adjuster comprising a hydraulic chamber defined within the cylinder of the caliper body by the cylinder and the piston, a braking member for engaging the piston to allow the piston to be restrained from rotating but movable in the axial direction, and an adjusting bolt and an adjusting nut to be in multi-threaded engagement therewith, said bolt and nut being disposed within the hydraulic chamber. The adjusting nut is engaged with the piston and thereby restrained from rotating, and is biased in the piston direction by means of a first spring, while the adjusting bolt is rotatable with respect to the cylinder and biased rearwardly of the cylinder by means of a second spring. The adjusting bolt is rotatably engaged at the end thereof in the rear of the cylinder with a push rod. The push rod intersects the adjusting bolt, and is restrained from rotating with respect to a cam shaft rotatably mounted at the rear portion of the cylinder. The rotation of the cam shaft can interchangeably be converted to a propelling force in the braking direction.
In the brake disc device of such a structure, however, there is a large change in the foot pressure to be applied upon the pedal of adjusting the hydraulic pressure, since larger fluctuations in the hydraulic pressure are incurred in the combined hydraulic and mechanical operation than in the hydraulic operation alone. Since the portion of the adjusting bolt to be in engagement with the push rod is exposed to air, there may be a reduction in the service life of a lubricating oil, etc. used in the clutch face defined at the portion where the adjusting bolt abuts upon the push rod, so that the friction resistance of the clutch face is apt to change. Furthermore, a problem arises in connection with the prevention of overadjustment due to the fact that the clutch face is of a small radius.
During charging of a working liquid, the operation of removal of air from the hydraulic chamber is troublesome.